Circulating Fluidized Bed Boiler

ABSTRACT

A circulating fluidized bed boiler includes a furnace for combusting carbonaceous fuel, at least one outlet channel connected to the upper portion of the furnace for removing flue gas and solid particles generated in the combustion of the fuel, each outlet channel provided with a particle separator attached with a flue gas channel for transferring cleaned flue gas and a return duct for transferring separated solid particles to the lower portion of the furnace. The return duct is provided with a gas seal, a heat exchange chamber, a lift channel and an overflow conduit, in which solid particles exiting the gas seal are guided to the upper portion of the heat exchange chamber and from the lower portion of the heat exchange chamber through the lift channel to the furnace, or directly from the upper portion of the heat exchange chamber through the overflow conduit to the furnace. At least one downpipe is connected from the upper portion to flow connection with the upper portion of the lift channel and from the lower portion to flow connection with the lower portion of the furnace. Additionally, the overflow conduit is connected to the upper portion of the downpipe. The downpipe is also preferably connected to an inlet for fuel.

This application is a U.S. national stage application of PCTInternational Application No. PCT/FI2009/050896, filed Nov. 6, 2009,published as PCT Publication No. WO 2010/052372 A1, on May 14, 2010, andwhich claims priority from Finnish patent application numberFI-20086055, filed Nov. 6, 2008.

BACKGROUND

The present invention relates to a circulating fluidized bed boilercomprising a furnace for combusting carbonaceous fuel, at least oneoutlet channel connected to the upper portion of the furnace forremoving flue gas and solid particles generated in the combustion of thefuel from the furnace, each outlet channel being provided with aparticle separator attached with a flue gas channel for transferringflue gas cleaned of solid particles out of the boiler, and a return ductfor transferring separated solid particles to the lower portion of thefurnace, the return duct being provided with a gas seal, a heat exchangechamber, a lift channel and an overflow conduit, wherein solid particlesexiting the gas seal are guided to the upper portion of the heatexchange chamber and from the lower portion of the heat exchange chamberthrough the lift channel to the furnace, or directly from the upperportion of the heat exchange chamber through the overflow conduit to thefurnace.

The circulating fluidized bed boiler of the present invention maypreferably be a large natural circulation boiler or a once throughboiler, for example, for power generation or industrial steamproduction. As the size of the boiler increases, the relation of thewall surface area to the volume of the furnace usually becomesdisadvantageous, which may cause problems, for example, in controllingthe boiler, positioning of the different devices and conduits related tothe furnace, as well as feed and mixing of different materials. Thepresent invention especially relates to solving problems related tolarge circulating fluidized bed boilers.

U.S. Pat. No. 7,240,639 discloses a circulating fluidized bed boiler, inwhich hot solid particles separated by a solids separator are guidedfrom a gas seal of a return duct to an upper portion of a heat exchangechamber integrated with the lower portion of the furnace of the boiler,and further, either from the lower portion of the heat exchange chamberto the furnace through a riser, or directly from the upper portion ofthe heat exchange chamber to the furnace through a separate overflowconduit. The heat exchange chamber is a fluidized chamber, which meansthat there are means provided in the lower portion of the chamber,especially, nozzles and inlet piping for fluidization gas, by means ofwhich the bed of solid particles being formed in the chamber can befluidized.

The heat exchange efficiency of a fluidized heat exchange chamber may beadjusted to a certain extent by changing the fluidizing velocity, inother words, the flow velocity of the fluidizing gas introduced to thechamber. However, an overflow channel offers another, very efficient wayof adjusting the heat exchange efficiency, by changing the share of thesolid particles removed from the heat exchange chamber as overflow.

The problem in the arrangement disclosed in U.S. Pat. No. 7,240,639 isthat the return ducts for cooled and uncooled particles, as well as theseparate fuel feeding channels, make the arrangement complicated andspace consuming by the walls in the lower portion of the furnace.According to a preferred embodiment disclosed in U.S. Pat. No.7,240,639, there are two separate lift channels connected to the heatexchange chamber, which improves the homogeneous distribution of thecooled solid particles and thus, promotes efficient, low-emissioncombustion of fuels.

U.S. Pat. No. 5,682,828 discloses a so-called divided gas seal that isformed to a return duct of a particle separator, in which gas seal thevertical return duct of the particle separator is connected to one endof a short fluidized horizontal duct, the other end of the ductconnecting to the lower portion of an upflow leg. The upflow leg is afluidized, at least mainly vertical channel, in which solid particlesare transferred from the bottom of the channel to the upper portionthereof by means of a sufficiently intense fluidizing gas flow.According to U.S. Pat. No. 5,682,828, a crosswise duct is connected tothe upper portion of the upflow channel, with an inclined downflowchannel leading from both ends thereof directly to furnace. The problemwith the solution disclosed in this patent is that the solid particlesto be returned can, for example, due to partial clogging, be drifted toreturn asymmetrically in such a way that the mass flow of one branch isgreater than that of the other branch.

U.S. Pat. No. 6,923,128 discloses a divided gas seal, which is similarto that of U.S. Pat. No. 5,682,828, except that the pipe connecting tothe upper portion of the upflow channel is bent in the middle, and withthe branches thereof pointing downwards inclined, and inclined towardsthe furnace. Furthermore, the upper end of the vertical portionconnecting to the branches is connected with a vertical inlet conduitfor fuel. A common problem with the return duct arrangements integratedwith the gas seals is that it is not possible to arrange a heat exchangechamber comprising an overflow duct in connection therewith.

An object of the present invention is to provide a circulating fluidizedbed boiler, in which the problems of the prior art discussed above areminimized or eliminated.

A special object of the present invention is to provide a compact andefficient circulating fluidized bed boiler, having a heat exchangerchamber provided in the return duct of a particle separator, the heatexchange efficiency of which can be adjusted and from which the solidparticles are evenly distributed throughout the furnace area in allconditions.

In order to solve the above-mentioned problems of the prior art, acirculating fluidized bed boiler is provided. A characterizing featureof a circulating fluidized bed boiler in accordance with the presentinvention is that there is at least one downpipe arranged in connectionwith the lift channel, the downpipe being connected at its upper portionto flow connection with the upper portion of the lift channel and at itslower portion to flow connection with the lower portion of the furnace,and the overflow conduit is in direct flow connection with the upperportion of the downpipe.

The term downpipe refers to a mainly vertical or almost vertical pipe orchannel, through the lower portion of which material flows directly tothe lower portion of the furnace. The downpipe is preferably dimensionedin such a way that in normal operation, only at the end of the downpipeconnecting to the furnace is there a short column of flowing material,and from the majority of the channel, the material quickly fallsthrough, whereby the channel is mainly almost empty.

The term overflow conduit refers to an opening, short channel or otherdirect connection from the upper portion of the heat exchange chamber tothe upper portion of the downpipe. The overflow conduit is arranged tojoin with the upper portion of the heat exchange chamber in such a waythat when the surface of the fluidized bed in the heat exchanger is lowenough, the solid particles being returned from the particle separatordo not pass through the overflow conduit to the downpipe, but throughthe heat exchange channel from up downwards, through an opening orconduit in the lower portion of the heat exchange chamber to the lowerportion of the lift channel, and further, through the upper portion ofthe lift channel to the downpipe. Thereby, the solid particles have tomove in the proximity of the heat exchange surfaces of the heat exchangechamber, whereby they cool down when releasing heat to the heat exchangemedium circulating on the heat exchange surfaces. Correspondingly, whenthe surface of the fluidized bed maintained in the heat exchange chamberis high enough, a portion of the solid particles being returned from theparticle separator passes from the upper portion of the heat exchangechamber through the overflow conduit, directly to the downpipe, withoutcooling on the heat exchange surfaces of the heat exchange chamber.

In some cases, it is possible to have only one downpipe connected to thelift channel, but according to a most preferred embodiment of thepresent invention, there are two downpipes at the same level connectingto one lift channel, the upper portions of both being connected by anoverflow conduit directly to the upper portion of the heat exchangechamber. The two downpipes are preferably arranged to different sides ofthe lift channel in such a way that the lift channel and the upperportions of the downpipes are successively in the direction of theclosest wall of the furnace. In some cases, it may be advantageous toconnect more than two downpipes to one lift channel, whereby there ispreferably an overflow conduit connecting to the upper portion of eachdownpipe. According to a preferred embodiment, the lift channel isconnected with two downpipes, of which one is connected with an overflowconduit, but the other one is not.

In some preferred embodiments of the invention, one heat exchangechamber can be connected with two or more lift channels, each of whichbeing possibly connected with one or more lift channels. According to amost preferred embodiment, the heat exchange chamber is connected withtwo lift channels, each channel being connected to one downpipe.Thereby, the lift channels are preferably located on the opposite sidesof the heat exchange chamber in such a way that the furnace side portionof the heat exchange chamber, the lift channels and the upper portionsof the downpipes connected thereto are adjacently positioned parallel tothe closest furnace wall. By increasing the amount of the downpipes, itis possible to achieve a very homogeneous distribution of the particlesreturned to the furnace.

The arrangement in accordance with the present invention, in which onelift channel is connected with two downpipes differs from thearrangements disclosed in U.S. Pat. No. 5,682,828 and U.S. Pat. No.6,923,128, especially in that the flow of solid particles being returnedfrom the particle separator is not divided in the lift channel of thegas lock, but in the lift channel of the heat exchange chamber arrangeddownstream of the gas seal, and, therefore, it is possible to connectoverflow conduits to the downpipes, and the downpipes can act indifferent situations either as discharge channels for material cooled onthe heat exchange surfaces or as overflow channels for uncooledmaterial.

As cooled and uncooled solid particles in the arrangement in accordancewith the present invention are returned to the furnace along the samedownpipe, the arrangement is simple, and surface area-saving for thewalls of the lower portion of the furnace. When several downpipes areused, it is possible to distribute both the cooled and uncooledparticles in the arrangement in accordance with the inventionhomogeneously to the furnace area.

When the heat exchange efficiency of the heat exchange chambers isadjusted, it is possible to preferably use such fluidizing velocities bywhich a portion of the solid particles arriving from the particleseparator are drifted via the heat exchange surfaces through the heatexchange chamber and another portion of the solid particles are directlyremoved from the upper portion of the heat exchange chamber via overflowconduits to the downpipes. Thereby, the downpipes act simultaneouslyboth as discharge channels for cooled material and overflow channels foruncooled material.

The combining of two material flows in the downpipe, as performed in theadjustment method disclosed above, preferably takes place without theflows disturbing each other, when the flow direction of both materialflows in the connection point of the flows is downwards. Thus, theoverflow conduit is preferably a short, downwards directing channel.Furthermore, the connection point of the lift channel and the downpipespreferably has to be, to a certain extent, at a lower level than theconnection points of the overflow conduits and downpipes. The connectionof the material flows is also facilitated by the fact that the particlesrapidly fall down from the upper portion of the downpipes to the lowerportion thereof, and in normal operating conditions, the downpipes arealmost empty in the connection point of the material flows.

Another meaning of the overflow conduits in accordance with theinvention is that they also offer an exit route to the lower portion ofthe furnace for the fluidizing air of the heat exchanged chamber,whereby there is no need to arrange a special exit to the furnace, or itcan be smaller than a conventional one, or it is not necessary to adjustthe pressure conditions of the return duct in such a way that thefluidizing air exits through the gas seal to the particle separator.Since there is a counter pressure prevailing in the lower portion of thefurnace, the level of which depends on the combusting conditions of thefurnace, the fluidizing air exits through the downpipes to the furnace,of course, only when the pressure level after the gas seal in the returnduct has reached a sufficient level.

Another advantage resulting from the feature that, for example, whenusing two downpipes, an overflow conduit has been arranged to the upperportion of both downpipes, is that the fluidizing gases exiting throughthe overflow conduits are guided to both downpipes, and, thus, they helpto a certain extent the directing of the solid particles being returnedfrom the particle separator evenly to both downpipes. An evendistribution of the particle flow to the downpipes and therealong to thewhole furnace area, as homogeneously as possible, facilitates theefficient burning of the fuel and enables the minimization of theemissions generated in the combustion.

The heat exchange chamber, the lift channel, and the upper portions ofthe downpipes preferably form an integral entity. In other words, theheat exchange chamber, the lift channel, and the upper portions of thedownpipes are formed attached to each other in such a way that theadjacent parts have at least mainly common partition walls. According toa most preferred embodiment, there are two downpipes connected to a liftchannel, and they are attached firmly to each other in such a way thatthe lift channel and the downpipes positioned on both sides thereof forman integrated structure, which is parallel to the closest wall of thefurnace.

The heat exchange chamber, the lift channel and the downpipes may bemanufactured as being completely or partially uncooled, in other words,plate-structured, and from the inside refractory lined structure.However, the heat exchange chamber, the lift channel and at least theupper portions of the downpipes are preferably made at least mainly as awater tube construction, which is connected to the water or steam cycleof the boiler. The particle separator and the gas seal are alsopreferably of water tube constructions, and made as an integrated watertube construction with the heat exchange chamber, the lift channel andat least the upper portions of the downpipes.

According to a preferred embodiment of the invention, at least onedownpipe is connected with an inlet for fuel. Preferably, all inlets forfuel that are required on the furnace walls comprising particleseparators are connected to the downpipes connecting to the return ductsof the particle separators and no separate inlet openings for fuel arenecessary. In this way, the overall area of the walls is saved, as thetotal number of the inlet openings required on the walls decreases.Advantageously, it is possible to arrange such a large number ofdownpipes comprising an inlet for fuel that a sufficiently homogeneousdistribution of fuel in the furnace is achieved.

Distribution and mixing of fuel in the furnace are also facilitated byintroducing the fuel to the furnace mixed in an already relatively largesolid particle flow. The fuel also dries and becomes warmer while mixingwith a hot solid particle flow, which expedites the ignition of the fueland its burning in the furnace.

The downpipe preferably comprises a non-vertical portion for introducingthe fuel, which is connected with a mainly vertical inlet for fuel.According to a preferred embodiment, two downpipes are connected to thelift channel of the heat exchange chamber, which both comprise anon-vertical portion connected with a vertical inlet for fuel. Such anon-vertical portion may be in the upper portion of the downpipe,especially, at the highest point of the downpipe, whereby, the inlet forfuel is preferably connected to the ceiling of the front end of thedownpipe.

According to another preferred embodiment, the inlet for fuel isconnected to the center portion of the downpipe, in other words, to aportion of the downpipe, which is positioned at least to some extentlower than the highest point of the mainly almost vertical downpipe,which portion is less steeply sloping than its surroundings, in otherwords, to a more non-vertical portion. The advantage in the arrangementis that the fuel is introduced to a portion of the downpipe, in whichthe solid particles being returned from the particle separator fall at arelatively high velocity towards the furnace, whereby, the risk ofclogging being generated in the connecting point of the fuel inlet isvery small.

The inlet for fuel connected to the downpipe is preferably at leastmainly a vertical or an almost vertical drop leg. Such a drop leggenerally comprises a blower, by means of which the flow of the fuel inthe drop leg is ensured. According to the present invention, thefluidizing gas in the heat exchange chamber exits to the furnace atleast partially through the downpipes, whereby the fluidizing gas alsoensures the flow of the fuel to the furnace.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is discussed in more detail below with referenceto the accompanying drawings, in which:

FIG. 1 schematically illustrates a circulating fluidized bed boiler inaccordance with a preferred embodiment of the present invention;

FIG. 2 schematically illustrates a vertical cross section of a preferredarrangement of a lift channel and downpipes connected therewith;

FIG. 3 schematically illustrates a vertical cross section of a secondpreferred arrangement of a lift channel and downpipes connectedtherewith.

FIG. 4 schematically illustrates a vertical cross section of a preferredarrangement of a heat exchange chamber, a lift channel and downpipesconnected therewith;

FIG. 5 schematically illustrates a vertical cross section of a secondpreferred arrangement of a heat exchange chamber, lift channels anddownpipes connected therewith;

FIG. 6 schematically illustrates a horizontal cross section of otherpreferred arrangements of a heat exchange chamber, lift channels anddownpipes connected therewith.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a circulating fluidized bed boiler 10 in accordancewith a preferred embodiment of the invention, having a furnace 12, anoutlet channel 14 for flue gas and solid particles entrained therewith,a particle separator 16, a flue gas channel 18 for transferring cleanedflue gas out of the boiler and a return duct 20, along which at least aportion of the particles separated in the particle separator 16 arereturned to the lower portion of the furnace 12. A gas seal 22 forpreventing the gas from flowing from the furnace 12 through the returnduct 20 to the particle separator 16 and a fluidized heat exchangechamber 24 are arranged in the return duct 20.

The furnace 12 is connected with conventional devices, such as aso-called wind box and fluidizing nozzles, for introducing primarycombustion gas 26 acting as fluidizing gas to the bottom of the furnace12 and means for introducing secondary combustion gas 28 to a certainextent higher level. The fluidizing gas and the combustion gases areusually air, but they can also comprise circulated flue gas and/oroxygen, or a mixture thereof. The walls of the furnace 12 may alsocomprise conventional conduits 30 for introducing fuel, bed material andsulfur-binding material to the furnace 12. Walls 32 of the furnace 12are usually made as water tube constructions in such a way that thewater tube walls in the lower portion of the furnace 12 are internallylined with refractory material.

There are usually different heat exchangers in the flue gas channel 18,such as superheaters, reheaters, an economizer and a preheater forcombustion air. The flue gas channel 18 can also comprise differentcleaning devices for flue gas, for example, dust separators as well asdevices for removing nitrogen oxides and sulfur dioxide. As thesedevices are not of importance for the present invention, they are notshown in FIG. 1.

The furnace 12 of a large circulating fluidized bed boiler 10 hasgenerally a rectangular horizontal cross section, having two long sidesand two short sides. In a large circulating fluidized bed boiler, thereare usually two, three or four particle separators, adjacently locatedon one long side of the furnace, or four, six or eight particleseparators, adjacently arranged on both long sides of the furnace 12.Thus, though FIG. 1 shows, as an example, only one particle separator16, it is to be understood that it is possible to have more particleseparators, and they can be arranged on both sides of the furnace 12.

When using a circulating fluidized bed boiler 10, solid particles havinga high temperature, which are separated by a particle separator 16, aretransferred from gas seal 22 to the upper surface of the fluidized bedof the heat exchange chamber 24. There is a lift channel 34 inconnection with the heat exchange chamber 24, having a bottom opening 36in the lower portion thereof, through which opening it is possible totransfer the solid particles from the lower portion of the heat exchangechamber 24 to the lift channel 34. In the upper portion of the liftchannel 34, there is a top opening 38, through which it is possible totransfer by means of fluidizing gas flow being fed through fluidizingmeans 40 of the lift channel 34 solid particles to the upper portion ofa downpipe 42 arranged in connection with the lift channel 34. As can beseen in the vertical cross sections of the lift channel 34 and downpipes42 in FIGS. 2 and 3, downpipes 42 can preferably be arranged to bothsides of the lift channel 34. The solid particles are preferably guidedfrom the lower portion of the downpipe 42 directly to lower portion ofthe furnace 12.

An overflow conduit 44 is arranged between the upper portion of the heatexchange chamber 24 and the upper portion of each downpipe 42, throughwhich the fluidizing gas of the heat exchange chamber 24 is allowed toexit to downpipes and therethrough to the furnace 12. The overflowconduit 44 is preferably connected to the downpipe 42 at a level whichis to a certain extent higher than the top opening 38 of the liftchannel 34. The fluidizing gas exiting from the overflow conduit 44assists in maintaining the downpipes 42 open. At the same time, thefluidizing gas of the heat exchange chamber 24 exiting through eachdownpipe 42 facilitates in distributing the solid material homogeneouslyto individual downpipes 42 and also to the area of the whole furnace 12.

The heat exchange chamber 24 has heat exchange surfaces 46, for example,superheater surfaces of the boiler 10, by means of which it is possibleto transfer heat from the hot solid particles separated by the particleseparator 16 to the heat exchange medium, for example, steam to besuperheated. The heat exchange efficiency can be adjusted to a certainextent by changing the fluidizing velocity of the heat exchange chamber24, in other words, the flow velocity of the fluidizing gas beingintroduced through fluidizing means 48 to flow through the heat exchangechamber 24. Another method of adjusting the heat exchange efficiency isto adjust the fluidizing velocity of the lift channel 34 in such a waythat only a portion of the solid particles separated by the particleseparator 16 exits from the heat exchange chamber 24 through the liftchannel 34. Thereby, the surface of the fluidized bed in the heatexchange chamber 24 rises in the upper portion of the chamber 24 to thelevel of the overflow conduits 44 and solid particles also start to exitthrough overflow conduits 44. As can be seen in FIGS. 2 and 3, overflowconduits 44 guide separated solid particles directly to the upperportion of the downpipes 42.

When the fluidizing velocity of the lift channel 34 is sufficient, allseparated hot solid particles exit from the bottom of the heat exchangechamber 24 through the lift channel 34. Thereby, as large an amount ofhot particles as possible also passes in the vicinity of the heatexchange surfaces 46, and the heat exchange efficiency of the heatexchange chamber 24 is maximal. The other extreme is to fluidizematerial in the lift channel 34 with such a low efficiency that allmaterial exits through overflow conduits 44, whereby the heat exchangeefficiency of the heat exchange chamber 24 is at its minimum. A thirdalternative is to use such a fluidizing velocity in the lift channel 34,by which a portion of the material exits through the lift channel 34 andanother portion through the overflow conduits 44. This way, the heatexchange efficiency can be adjusted continuously and accurately.

An inlet for fuel 50, preferably, a vertical or partially inclined dropleg, is connected to the ceiling of downpipes 42, as shown in FIG. 2.The drop leg is preferably connected to an inlet for air 52, throughwhich it is possible to introduce air or other gas to promote the flowof the fuel downwards. When the fuel enters the downpipe 42, it mixeswith the flow of the solid particles to be returned. The solid particleflow, which is warmer than the fuel, dries and preheats the fuel in sucha way that having entered the furnace 12, it ignites and burns rapidly.At the same time, the gas exiting the overflow conduits 44 intensifiesthe flow of the fuel to the furnace 12.

FIG. 3 shows another arrangement that differs from that disclosed inFIG. 2, in having the inlets for fuel 50 connected to a portion 54,which is arranged to the center portion of the downpipes 42, in otherwords, to at least somewhat lower a level than their highest point,which is less steeply sloping than the surrounding portions. Theadvantage in this arrangement is that the flow of solid material beingreturned from the heat exchange chamber 24 and falling along thedownpipe has at the inlet for fuel 50 already reached a relatively highvelocity, whereby, it will efficiently draw the fuel falling from theinlet 50 with it.

FIG. 4 shows a schematic horizontal cross-sectional view of anarrangement of a heat exchange chamber 24 of a circulating fluidized bedboiler 10, a lift channel 34 connected thereto and downpipes 42connected to the lift channel 34 in accordance with the invention,corresponding to the arrangement of FIG. 3. As can be seen from FIG. 4,the lift channel 34 and the downpipes 42 are successively arrangedparallel to the closest side wall 32′ of the furnace 12. Thus, the heatexchange chamber 24, lift channel 34 and downpipes 42 form a compactentity, which can advantageously be connected to the fuel transfersystem and can be arranged, when necessary, very close to the furnace12.

In the arrangement in accordance with FIG. 4, non-vertical portions 54in the center portion of the downpipes 42 are directed along the sidewall 32′ of the furnace 12, and distributing conveyors 56 transferringfuel to the inlets for fuel or to drop legs, such as robbing screws,bring fuel from a common conveyor 58, for example, a flight conveyor,preferably, past the heat exchange chamber 24, perpendicularly to theside wall 32′ of the furnace. Thereby, the common conveyor 58 bringingfuel to a number of distributing conveyors 56 can preferably be locatedbehind the heat exchange chamber(s) 24, as seen from the furnace 12.Thus, in the arrangement in accordance with the invention, it is notnecessary to locate fuel conveyors between the heat exchange chamber(s)24 and the furnace 12.

In the furnace 12 of the circulating fluidized bed boiler 10, inaccordance with the present invention, the particle separators 16 andheat exchange chambers 24 connected thereto, and the devices andchannels therebetween, are typically all structures hung from the solidsupporting structure of the boiler 10. Especially with large boilers, itis possible to make considerable savings by keeping the supportingstructure as small as possible. Therefore, a compact arrangement of theheat exchange chamber 24 and the channels connected thereto, inaccordance with the invention, close to the furnace, significantly savesin the total construction costs of the whole power plant.

FIG. 5 shows a schematic horizontal cross-sectional view of a secondarrangement of a heat exchange chamber 24, a lift channel 34 connectedthereto and downpipes 42 connected to the lift channel 34 in acirculating fluidized bed boiler 10 in accordance with the invention.The arrangement of FIG. 5 differs from that of FIG. 4 in that there aretwo lift channels 34 connected to the heat exchange chamber 24, one oneach side of the heat exchange chamber 24. The downpipes 42 connected tothe lift channels are adjacent to the lift channels 34 parallel to theclosest side wall 32′ of the furnace 12. Thus, in the arrangementillustrated in FIG. 5, the heat exchange chamber 24, lift channels 34and the upper portions of the downpipes 42 are successively arrangedparallel to the closest side wall 32′ of the furnace 12.

The inlets for fuel 50 in the arrangement shown in FIG. 5 are connectedto the ceiling of the downpipes 42, in a similar way as in thearrangement of FIG. 2. The arrangement of lift channels 34 and downpipes42 also can, however, be modified in such a way that the inlets for fuel50 are connected to a less steeply sloping point in the center portionof the downpipes 42, as in the arrangement of FIG. 4. The arrangement ofFIG. 5 is especially advantageous, as the heat exchange chamber 24, thelift channels 34 and the downpipes 42 there form an especially compactentity in a direction perpendicular to the closest wall 32′ of thefurnace 12, which can be arranged, if necessary, especially close to thefurnace 12. The disadvantage of the arrangement is that as the downpipes42 are spaced apart from the heat exchange chamber 24, the overflowconduits leading from the upper portion of the heat exchange chamber 24to the upper portion of the downpipes 42 cannot be mere openings, butthey must be separate, short channels 44′.

FIG. 6 shows yet another way of locating a heat exchange chamber 24, twolift channels 34 and downpipes 42 connected thereto. In the arrangementin accordance with FIG. 6, the lift channels 34 and the downpipes 42 arearranged in connection with the walls of the heat exchange chamber 24perpendicular to the closest side wall 32′ of the furnace 12. Thus, thedistance between the heat exchange chamber 24 and the furnace 12 can bekept short, and the overflow conduit 44 is a mere opening in the wallbetween the heat exchange chamber 24 and the downpipe 42. This figurealso illustrates two different ways of locating the inlet for fuel 50 tothe portion 54 less steeply sloping than the surrounding downpipe 42.The upper portion of FIG. 6 shows an arrangement in which the lesssteeply sloping portion 54 of the downpipe 42 is directed parallel tothe side wall 32′ of the furnace 12, whereby the distance of the heatexchange chamber 24 from the furnace 12 can be minimized.Correspondingly, the lower portion of FIG. 6 shows an arrangement, inwhich the less steeply sloping portion 54 is directed towards thefurnace 12, whereby the structure is to a certain extent simpler andnarrower in the direction of the side wall 32′ of the furnace 12.

The invention has been described above in connection with exemplaryarrangements, but the invention also comprises various combinations ormodifications of the disclosed embodiments. Especially, the number andgeometry of the lift channels and downpipes may vary from what is shownin FIGS. 1-6. Thus, it is obvious that the exemplary embodimentsdisclosed herein are not intended to limit the scope of the invention,but several other embodiments are also included in the invention, theseembodiments being limited only by the appended claims and thedefinitions therein.

1. A circulating fluidized bed boiler comprising: a furnace forcombusting carbonaceous fuel; and at least one outlet channel connectedto the upper portion of the furnace for removing flue gas and solidparticles generated in the combustion of the fuel from the furnace, eachoutlet channel being provided with a particle separator attached with aflue gas channel for transferring flue gas cleaned of solid particlesout of the boiler and a return duct for transferring separated solidparticles to the lower portion of the furnace, said return duct beingprovided with (i) a gas seal, (ii) a heat exchange chamber, (iii) a liftchannel and (iv) an overflow conduit, wherein solid particles exitingthe gas seal are guided to the upper portion of the heat exchangechamber and from the lower portion of the heat exchange chamber throughthe lift channel to the furnace or directly from the upper portion ofthe heat exchange chamber through the overflow conduit to the furnace,wherein at least one downpipe is arranged in connection with the liftchannel, the downpipe being connected at its upper portion to flowconnection with the upper portion of the lift channel and at its lowerportion to flow connection with the lower portion of the furnace, andthe overflow conduit is in direct flow connection with the upper portionof the downpipe.
 2. A circulating fluidized bed boiler according toclaim 1, further comprising downpipes arranged on two separate sides ofthe lift channel, said downpipes being provided with an overflow conduitin such a way that the lift channel and the upper portions of thedownpipes are successively in the direction of the closest wall of thefurnace.
 3. A circulating fluidized bed boiler according to claim 1,wherein two opposite sides of the heat exchange chamber are providedwith a lift channel, said lift channels being connected with arespective downpipe and said downpipes being connected with an overflowconduit in such a way that the heat exchange chamber, the lift channels,and the upper portions of the downpipes are successively in thedirection of the closest wall of the furnace.
 4. A circulating fluidizedbed boiler according to claim 1, further comprising a lift channel and adownpipe arranged in connection with two side walls of the heat exchangechamber, said side walls being perpendicular to the closest wall of thefurnace, and said sidewalls being provided with an overflow conduit. 5.A circulating fluidized bed boiler according to claim 1, wherein theheat exchange chamber, the lift channel or lift channels, the upperportions of the downpipes and the overflow conduits form an integralentity.
 6. A circulating fluidized bed boiler according to claim 5,wherein the heat exchange chamber, the lift channel or lift channels,the upper portions of the downpipes and the overflow conduits are mainlymade as water tube constructions.
 7. A circulating fluidized bed boileraccording to claim 1, wherein the overflow conduit is connected to thedownpipe at a higher level than the lift channel.
 8. A circulatingfluidized bed boiler according to claim 1, wherein at least one downpipeis provided with an inlet for fuel.
 9. A circulating fluidized bedboiler according to claim 8, wherein the inlet for fuel is connected toa non-vertical portion of the downpipe.
 10. A circulating fluidized bedboiler according to claim 9, wherein the non-vertical portion forms aceiling of the downpipe.
 11. A circulating fluidized bed boileraccording to claim 9, wherein the non-vertical portion forms an inclinedportion below the highest point of the downpipe.
 12. A circulatingfluidized bed boiler according to claim 8, wherein the inlet for fuel isa drop leg.
 13. A circulating fluidized bed boiler according to claim 8,wherein the inlet for fuel is attached to a distributing conveyor fortransferring fuel, said conveyor being directed alongside the heatexchange chamber directly towards the furnace.
 14. A circulatingfluidized bed boiler according to claim 13, wherein the distributingconveyor is in connection with a common conveyor for transferring fuel,said common conveyor being aligned with the closest wall of the furnace,behind the heat exchange chamber, as viewed from the furnace.